Our objective is to increase our understanding of the mechanisms of implantation by studies of the changes in the surface coats of trophoblast cells,the processes by which trophoblast cells penetrate uterine epithelium, and maturation of blastomeres into trophoblast. Study of the surface coats will involve relationship to protein uptake and to adhesion, and will be examined by cytological, enzyme digestion, and agglutination methods. The mechanisms of implantation will be studied by combined scanning and transmission electron microscopy, and by development of an in vitro implantation model. In addition, the relationship of uterine secretion to the implantation process is being studied using radioautographic methods.